1) Field of the Invention
Embodiments of the present invention relate to the inspection of a workpiece and, more particularly, to apparatus and methods for providing two-dimensional and three-dimensional information indicative of a workpiece.
2) Description of Related Art
Composite structures are commonly manufactured by progressively building up the structure with a plurality of layers of thin composite tape (or tow) laid one layer upon another. Typically, the operation begins by laying one or more tapes onto a starting template or tool that has a configuration generally corresponding to the desired shape of the article to be produced. A tape placement head of a manufacturing system moves over the surface of the template, guiding the one or more tapes of composite material onto the template. The head usually makes repeated passes over the template in a defined pattern until the composite material is entirely collated, building up successive layers of the composite tape to form the desired workpiece. A compaction roller is typically used for pressing the tape against the workpiece, thereby facilitating adhesion of the successive layers. The workpiece may then be subjected to a curing process (e.g., heating) to further adhere and bond the composite layers. Conventional systems for forming composite structures using successive layers of tape include those systems disclosed, for example, in U.S. Pat. No. 6,799,619 issued to Holmes et al. and U.S. Pat. No. 6,871,684 issued to Engelbart et al.
The measurement accuracy required by the manufacturing specification, which is in turn driven by design requirements, in areas such as ply boundaries, tape edge gaps and overlaps, material wrinkles, and the presence of foreign object debris (FOD), has created a need to make those measurements with a robust automated system. Prior and emerging art, using various machine vision technologies, have provided limited capabilities to meet these requirements. As such, manual visual inspection of composite plies is frequently employed, which may be unreliable, inefficient, and subject to operator error. Namely, the machine must be stopped and the process of laying materials halted until the inspection is complete. During the inspection, the operator verifies the dimensions of any suspect inconsistencies and quantifies the number of inconsistencies per given unit area. The inconsistencies are repaired as needed and laying of the next ply proceeds.
Vision systems have been developed that are capable of inspecting workpieces as tape is laid thereon. Typically, a laser projector is employed that generates a laser signature on the workpiece, while a camera is used to capture an image of the workpiece that includes the laser signature illuminated thereon. The original presumption was that the expected laser line signatures seen during normal manufacturing would be well-known, and any variance from a “perfect” signature constituted a detected inconsistency. Thus, operation of the camera required reasonably clean, straight laser signatures in a particular location in the image frame. This assumption proved unworkable in the real world once prototype systems were built and tested on an actual composite tape lay-up machine.
For instance, the optical appearance of the composite materials may not be as consistent as obtained from coupons that have been examined earlier in the laboratory and vary greatly in reflectivity degree and sensitivity to fiber orientation. In addition, the working distance from the camera to the workpiece varies significantly, which causes the laser signature to move completely through the vertical extent of the camera frame, rather than remaining near the centerline of the image height. The large motion of the workpiece also moves the surface outside of the focal depth of fields of both the laser projector and the camera. While the camera's depth of field is easily controlled by modifying the lens, the depth of field of the laser generator cannot be changed such that the imaged laser signature is often out of focus. Moreover, the actual working depth of field is at times transited at very high velocity (i.e., there are sharp “bumps” in the surface of the workpiece in addition to slowly-changed “swells”), which produces significant motion blur in the vertical image direction as the laser signature sweeps through the field. The surface of the workpiece is not always flat, resulting in laser signatures that are rarely straight lines. Furthermore, methods of FOD detection and classification that depend upon two-dimensional machine vision algorithms are defeated by the presence of a strong laser line signature.
It would therefore be advantageous to provide apparatus and methods for inspecting a workpiece to increase the reliability and accuracy of the inspection of the workpiece. In addition, it would be advantageous to provide apparatus and methods to increase the quality of a workpiece, the production rate, and inspection efficiency, as well as reduce the overall cost of the manufacturing and inspection processes.